Of course, his lessons didn’t change on the day of the attacks, but once students started showing up who had completely forgotten about it — “18-year-olds who were about seven when 9/11 happened” — he knew he had to teach it. But there are only so many hours of instruction in the semester.

That meant he had to start making cuts in his lesson plans. Take Watergate. Once, he used to spend an entire lecture on the political scandal, but now, he covers it in 10 to 15 minutes. “The New Deal is another really good example,” he says. “When I first started teaching, I think I had three lectures on it.” Now he’s down to two, and that’s changed the way he teaches, too. “I try to do it differently so that I won’t overwhelm people with lots and lots of facts. And then they’ll be able to understand more history, hopefully, through only having to know a little less.”